Abstract

The association constant for the reaction: Mg2+ + SO2-4 ⇌ MgSO4 has been measured at 25°C using magnesium ion-selective electrode (Mg-ISE) potentiometry in aqueous solutions of ionic strength (I) ranging from 0.25 to 6 M in CsCl and in 1 M (Me4NCl). The value of log β(MgSO4) = 0.98 ± 0.02 in 1 M (Me4NCl) was significantly higher than that of 0.75 ± 0.01 obtained in 1 M (CsCl). This difference can be explained by a weak association between Cs+ and SO2-4, with log β(CsSO-4) = - 0.11 ± 0.03, which is also qualitatively consistent with the absence of an increase in β(MgSO4) at high ionic strength in CsCl media. Extrapolation of the results in CsCl media gave an infinite dilution value of log β° (MgSO4) = 2.38 ± 0.03 that was rather dependent on the nature of the extrapolation function. The performance of the Mg-ISE in various media is also briefly described.